Unison members demonstrating against government spending cuts in 2011. The union said pay freezes and below-inflation rises had left pay reduced by almost 20% since the coalition came to power. Photograph: Paul Brown/Rex Features

More than half a million council workers and school support staff are to be balloted on strike action after they rejected a 1% pay offer.

Unison said 600,000 of its members in England would vote in the coming weeks on whether to launch a campaign of industrial action.

The move followed a consultative ballot which showed 70% of Unison members opposed a pay offer the union said was worth a below-inflation 1% for most workers.

Employees covered by the offer include teaching assistants, planners, administrators, social workers and engineers.

Unison said local government workers had endured three consecutive years of pay freezes, followed by below-inflation rises in 2013 and 2014, leaving their pay reduced by almost 20% since the coalition came to power.

National officer Heather Wakefield said: "Our members have made it clear that this pay offer is the straw that breaks the camel's back after years of pay freezes and below-inflation rises.

"This offer is effectively another pay freeze for the majority of our local government and school members, and they have used this consultation to send a strong message that it is insulting and unacceptable.

"Local government workers have kept services running in our communities in the face of the government's harsh austerity agenda, and they deserve more than just a bare minimum pay increase."